Fair Value Level 3 Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Trading securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	$ 2	$ 26	$ 2	$ 13
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	80	2,136	1,201	1,127
Total gains (losses) (realized/unrealized) Included in Net Income	(3)	14	77	32
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	1	63
Sales	0	0	(1,060)	(21)
Issues	0	0	0	0
Settlements	0	(9)	(6)	(34)
Transfers out of Level 3	0	(8)	(137)	(30)
Transfers into Level 3	0	7	1	1,003
Ending Balance	77	2,140	77	2,140
Trading securities [Member] | Fannie Mae [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	2	27	2	3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	79	1,871	971	835
Total gains (losses) (realized/unrealized) Included in Net Income	(3)	16	163	20
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	1	63
Sales	0	0	(1,060)	0
Issues	0	0	0	0
Settlements	0	0	0	(5)
Transfers out of Level 3	0	(8)	0	(30)
Transfers into Level 3	0	7	1	1,003
Ending Balance	76	1,886	76	1,886
Trading securities [Member] | Other agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)			0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance			35
Total gains (losses) (realized/unrealized) Included in Net Income			(1)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)			0
Purchases			0
Sales			0
Issues			0
Settlements			(1)
Transfers out of Level 3			(33)
Transfers into Level 3			0
Ending Balance	0		0
Trading securities [Member] | Alt-A and subprime private-label securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(1)	0	10
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		264	194	271
Total gains (losses) (realized/unrealized) Included in Net Income		(2)	(85)	9
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)		0	0	0
Purchases		0	0	0
Sales		0	0	0
Issues		0	0	0
Settlements		(9)	(5)	(27)
Transfers out of Level 3		0	(104)	0
Transfers into Level 3		0	0	0
Ending Balance	0	253	0	253
Trading securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1	1	1	21
Total gains (losses) (realized/unrealized) Included in Net Income	0	0	0	3
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	(21)
Issues	0	0	0	0
Settlements	0	0	0	(2)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	1	1	1	1
Available-for-sale securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,093	1,637	1,313	2,153
Total gains (losses) (realized/unrealized) Included in Net Income	8	5	23	41
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(11)	11	(51)	(32)
Purchases	0	0	0	0
Sales	(4)	(59)	(22)	(384)
Issues	0	0	0	0
Settlements	(50)	(91)	(223)	(254)
Transfers out of Level 3	0	(16)	(4)	(67)
Transfers into Level 3	0	8	0	38
Ending Balance	1,036	1,495	1,036	1,495
Available-for-sale securities [Member] | Fannie Mae [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	204	206	208	230
Total gains (losses) (realized/unrealized) Included in Net Income	0	0	1	1
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(1)	0	(1)	(2)
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(3)	(2)	(8)	(8)
Transfers out of Level 3	0	(16)	0	(63)
Transfers into Level 3	0	8	0	38
Ending Balance	200	196	200	196
Available-for-sale securities [Member] | Other agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)				0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance				5
Total gains (losses) (realized/unrealized) Included in Net Income				0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)				0
Purchases				0
Sales				(1)
Issues				0
Settlements				0
Transfers out of Level 3				(4)
Transfers into Level 3				0
Ending Balance		0		0
Available-for-sale securities [Member] | Alt-A and subprime private-label securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	26	178	77	217
Total gains (losses) (realized/unrealized) Included in Net Income	0	0	0	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	10	(45)	(5)
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(2)	(8)	(4)	(32)
Transfers out of Level 3	0	0	(4)	0
Transfers into Level 3	0	0	0	0
Ending Balance	24	180	24	180
Available-for-sale securities [Member] | Mortgage revenue bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	506	873	671	1,272
Total gains (losses) (realized/unrealized) Included in Net Income	1	5	1	40
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(3)	(3)	(6)	(15)
Purchases	0	0	0	0
Sales	(4)	(59)	(22)	(383)
Issues	0	0	0	0
Settlements	(36)	(67)	(180)	(165)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	464	749	464	749
Available-for-sale securities [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0	0	0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	357	380	357	429
Total gains (losses) (realized/unrealized) Included in Net Income	7	0	21	0
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	(7)	4	1	(10)
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(9)	(14)	(31)	(49)
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	348	370	348	370
Mortgage loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	2	6	17	21
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,018	1,119	1,116	1,197
Total gains (losses) (realized/unrealized) Included in Net Income	7	9	35	41
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(47)	(58)	(174)	(175)
Transfers out of Level 3	(44)	(6)	(131)	(73)
Transfers into Level 3	31	26	119	100
Ending Balance	965	1,090	965	1,090
Net derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	(27)	(14)	(56)	3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	117	124	134	44
Total gains (losses) (realized/unrealized) Included in Net Income	(18)	18	(104)	118
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	(1)	(16)	(15)	(40)
Transfers out of Level 3	0	0	53	5
Transfers into Level 3	0	(1)	0	(2)
Ending Balance	98	125	98	125
Long-term debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	7	(9)	26	(28)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(673)	(1,125)	(958)	(588)
Total gains (losses) (realized/unrealized) Included in Net Income	9	(6)	38	(28)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	1	(2)
Settlements	4	33	35	52
Transfers out of Level 3	172	141	503	229
Transfers into Level 3	(22)	(117)	(129)	(737)
Ending Balance	(510)	(1,074)	(510)	(1,074)
Long-term debt [Member] | Consolidated Trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	(5)	(3)	(6)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(319)	(760)	(582)	(241)
Total gains (losses) (realized/unrealized) Included in Net Income	2	(2)	9	(6)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	1	(2)
Settlements	4	33	35	52
Transfers out of Level 3	172	141	503	229
Transfers into Level 3	(22)	(117)	(129)	(737)
Ending Balance	(163)	(705)	(163)	(705)
Long-term debt [Member] | Senior floating [Member] | Fannie Mae [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	7	(4)	29	(22)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(354)	(365)	(376)	(347)
Total gains (losses) (realized/unrealized) Included in Net Income	7	(4)	29	(22)
Total gains (losses) (realized/unrealized) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issues	0	0	0	0
Settlements	0	0	0	0
Transfers out of Level 3	0	0	0	0
Transfers into Level 3	0	0	0	0
Ending Balance	$ (347)	$ (369)	$ (347)	$ (369)